The present invention relates to a method of producing a semiconductor device. In particular, the invention relates to a method of producing a semiconductor device including forming trenches in a semiconductor substrate and filling the trenches by an epitaxial growth method.
Heretofore, in a semiconductor device such as an MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor), a bipolar transistor, a diode, etc., when a region in which a drift current flows (hereinafter referred to a drift layer) is made thin, the current path of the drift current is shortened so that on-resistance becomes low but the withstand voltage becomes low. In contrast, when the drift layer is made thick, the withstand voltage becomes high but on-resistance becomes high. In this manner, this type of semiconductor device has a trade-off relationship between the on-resistance (current capacity) and the withstand voltage.
A super junction structure is commonly known as a technique for improving the trade-off relationship. FIG. 10 is a sectional view showing the super junction structure of the semiconductor device. As shown in FIG. 10, the super junction structure is a structure in which the drift layer is formed not as a single semiconductor layer, but instead, as a junction structure (hereinafter referred to as ‘parallel pn structure 4’) of alternate arrangement of a high impurity concentration n-type semiconductor region 2 and a high impurity concentration p-type semiconductor region 3. A method of forming a trench in an n-type drift layer by dry etching and filling the trench with a p-type semiconductor by epitaxial growth has been proposed as a method for forming the parallel pn structure 4.
A field oxide film, a gate oxide film and a gate electrode, which are not shown in FIG. 10, are formed successively in a surface layer of the parallel pn structure 4 by an ordinary MOSFET producing process, so that p base regions 5 are formed as shown in FIG. 10. On this occasion, it is necessary to form a p base region 5 in a surface layer of each p-type semiconductor region 3 accurately in order to obtain a required operation of the semiconductor device. It is therefore necessary to form a marker (hereinafter referred to as ‘alignment marker’) in the surface layer of the n-type drift layer before the formation of the parallel pn structure 4 so that the marker can serve as a reference for accurately aligning a semiconductor substrate with a photo mask disposed above the semiconductor substrate. When a mask pattern needs to be transferred, the alignment marker is recognized by an exposure apparatus or the like so that the photo mask can be disposed in an accurate position. Consequently, the desired mask pattern is transferred onto a surface of the parallel pn structure 4.
The following method has been proposed as a method for forming the aforementioned alignment marker. In a semiconductor device producing method including the step of flattening a surface of a semiconductor substrate by buffing in the middle of a wafer process, an alignment marker formed before the flattening step is shaped like an inverted taper in sectional view. On this occasion, the semiconductor substrate has alternate and parallel arrangement of a p layer and an n layer shaped like stripes perpendicular to a principal surface of the semiconductor substrate. Then, isotropic etching of silicon is used for forming an alignment hole-like marker shaped like an inverted taper in sectional view (see, for example, JP-A-2006-303232).
The following method has been proposed as another method for forming the aforementioned alignment marker. A first trench is formed in an n-type semiconductor substrate so that the first trench can serve as a target trench. The inside of the first trench and the surface of the semiconductor substrate are covered with a mask. The mask is partially removed from a region in which a second trench will be formed. The second trench is formed in the unmasked region of the semiconductor substrate. On this occasion, while a p-type semiconductor is epitaxially grown on the inside of the second trench in the condition that the depth of the first trench is set to be larger than one fifth of the depth of the second trench, p-type semiconductor regions are formed in a parallel pn junction structure. After removal of the mask, the surface of the semiconductor substrate is polished by a thickness corresponding to a value not larger than one fifth of the depth of the second trench. On this occasion, the mask is made of an oxide film (see, for example, JP-A-2004-063894).
However, an additional step only for the formation of the alignment marker as disclosed in JP-A-2006-303232 is required for forming the alignment marker in addition to the ordinary MOSFET producing process. In the technique disclosed in JP-A-2004-063894, the mask oxide film remains on the surface of the n-type drift layer when the trenches are filled with p-type semiconductors by epitaxial growth. When the epitaxial layers are grown in a state where the mask oxide film remains, there is a possibility that defects may be produced in the epitaxial layers of the p-type semiconductor region and the n-type semiconductor region by stress of the mask oxide film. There is a possibility that the defects may be a cause of occurrence of a leakage current.
In view of the above, it would be desirable to provide an efficient semiconductor device producing method for forming a device surface structure in a desired position of a surface layer of a semiconductor substrate in production of a semiconductor device having a super junction structure. It would further be desirable to provide a semiconductor device producing method which can reduce defects produced in an epitaxial layer when a super junction structure region is formed in a semiconductor device.